Extended Controllability of HVDC Converters in the Vector Space

This paper investigates the controllability of modular multilevel converters for high voltage direct current applications. The paper provides an in-depth analysis of a computationally efficient method to calculate the required amount of triplen harmonic injection (THI) and to devise an overmodulation strategy based on a three-dimensional representation of the converter control voltages in the vector space. The proposed method allows simultaneous converter operation and power quality control in an extended ac voltage range without degrading the submodules (SM's) capacitor voltage ripple or spectra of generated waveforms with respect to well-known THI methods. The advantages of the presented technique with respect to traditional THI techniques are: first, the converter controllability is extended during ac-side voltage disturbances; second, the SM's capacitor voltage ripple is upper bounded; and third, the insulation stress due to the common-mode voltage at the transformer's secondary windings is minimized. The performance of the claimed features is supported by simulation results for both steady-state and transient operating conditions.

I. Introduction

With the advent of high-voltage direct current (HVDC) converters in the power delivery industry, considerable attention has been paid to the development of converter topologies and control strategies in order to maximize the efficiency of the AC/DC-DC/AC conversion process. Moreover, the HVDC energy market will continue experiencing growth for the next 20 years [1], [2] at a rate of USD 2 bn/year [3]. For instance, the UK's National Grid set out its plan to develop new electricity transmission infrastructure in Great Britain with combined capital and operational expenditure of USD 21.3 bn for the period 2013 to 2021 [4]. While the HVDC market growth has been primarily enabled by the accelerated inception of the modular multilevel converter (MMC) [5], further improvements in HVDC converter technology such as a reduced converter size per unit power, simultaneous control of active and reactive power delivery, DC fault blocking capabilities and integration with multi-terminal DC transmission schemes offered by state-of-the-art topologies [6] such as alternate arm converter (AAC) [7] are expected to continue to intensify the investment in the HVDC transmission business.